1. Field of the Invention
The present invention relates to a method for producing an aqueous ethylene-vinyl alcohol copolymer (EVOH) composition, and to pellets of EVOH obtained according to the production method.
2. Discussion of the Background
EVOH is a useful polymer material having good oxygen barrier properties, aroma retentiveness, oil resistance, antistatic properties and mechanical strength, and is widely used for films, sheets, containers, etc. One general method of producing EVOH comprises saponifying an ethylene-vinyl ester copolymer that is obtained through polymerization of ethylene with a vinyl ester of a fatty acid such as vinyl acetate or the like, in an alcohol-containing organic solvent in the presence of a saponification catalyst.
For post-treating the alcohol solution of EVOH obtained through saponification, for example, Japanese Patent Publication No. 38634/1972 (U.S. Pat. No. 3,847,845) discloses a method of producing EVOH pellets which comprises adding water to an optionally-concentrated methanol solution of EVOH to such a degree that no EVOH deposit is formed therein to prepare a methanol-water mixed solution of EVOH that contains from 15 to 45% by weight of EVOH, then extruding it into strands in water or in a methanol-water mixed solution having a lower methanol concentration that the EVOH solution, at 50xc2x0 C. or lower, and thereafter cutting the strands. It is stated therein that the methanol concentration in the methanol-water mixed solution in the coagulating bath preferably is between 10 and 50% by weight. It is further stated therein that the pellets thus obtained are porous and can be readily washed with water to remove the saponification catalyst residue and that the pellets are easy to handle in the subsequent washing and drying step.
Regarding the process of adding water to a methanol solution of the EVOH after saponification, for example, demonstrated in Example 1 of Japanese Patent Laid-Open No. 90927/1999 (EP 937,557) is a method of producing a completely transparent, uniform methanol/water solution of EVOH, which comprises adding an aqueous methanol solution having a water content of 62.5% by weight to a methanol solution of EVOH having an EVOH content of 30% by weight, under an azeotropic condition at 100xc2x0 C. to 110xc2x0 C. under a pressure of 3 kg/cm2G so as to remove methanol until the EVOH content of the resulting EVOH solution increases up to 40% by weight.
However, in the method of producing EVOH pellets by coagulating an EVOH solution that contains a large amount of methanol, the alcohol often vaporizes in the step of coagulation. The alcohol vapor worsens the working environment and injures the health of the workers, and, in addition, it has negative influences on the area around the working site. Therefore, improving the method is desired.
In cases where EVOH having an ethylene content of smaller than 20 mol % and/or a degree of saponification of smaller than 95% is processed according to the method as above, EVOH strands are difficult to form in the coagulating bath since the coagulation rate of EVOH in the bath is low. As a result, the EVOH strands, even though formed, are often miscut and are often contaminated with fine powder. Stable production of EVOH pellets in the method is difficult. Even EVOH having an ethylene content of 20 mol % or more and/or a degree of saponification of 95% or more is poorly coagulated or is difficult to handle, when it is coagulated in strands at an increased coagulation rate through an increased number of nozzles so as to increase the production efficiency, and, as a result, its strands are often miscut and stable production of its pellets is not easy.
In addition, the EVOH pellets obtained in the conventional methods contain alcohol and the catalyst residue in saponification, alkali metal salts, and therefore require washing to remove them. To increase the washing speed, elevating the temperature of the washing liquid will be effective. However, if the temperature is elevated too high, the pellets will fuse as they contain alcohol. Therefore, it is a problem that the washing speed cannot be sufficiently increased.
Accordingly, one object of the present invention is to provide a method for producing an aqueous EVOH composition that ensures efficient removal of alcohol without worsening the environment in and around working areas. Another object is to provide EVOH pellets of the aqueous composition which can be produced stably and can be washed rapidly.
We, the present inventors have assiduously studied, and have found that the problems noted above can be solved by providing a method for producing an aqueous ethylene-vinyl alcohol copolymer composition that comprises contacting an alcohol-containing ethylene-vinyl alcohol copolymer solution with water vapor to thereby remove the alcohol along with water vapor.
Specifically, one aspect of the invention is a method for producing an aqueous ethylene-vinyl alcohol copolymer composition, which comprises introducing an ethylene-vinyl alcohol copolymer solution that contains at least 50 parts by weight, relative to 100 parts by weight of the ethylene-vinyl alcohol copolymer therein, of an alcohol having a boiling point of not higher than 100xc2x0 C., into a vessel, and contacting the solution with water vapor in the vessel, thereby letting the alcohol out along with water vapor and taking the resulting aqueous ethylene-vinyl alcohol copolymer composition that contains from 0 to 10 parts by weight of the alcohol and from 10 to 500 parts by weight of water, relative to 100 parts by weight of the ethylene-vinyl alcohol copolymer therein, out of the vessel.
In the production method, preferably, the ethylene-vinyl alcohol copolymer solution is continuously introduced into the vessel. Also preferably, the ethylene-vinyl alcohol copolymer solution is contacted with water vapor in a column vessel, or is contacted therewith in countercurrent flow.
In one preferred embodiment of the method, the ethylene-vinyl alcohol copolymer solution is introduced into the vessel through its upper part with water vapor through its lower part, and the aqueous ethylene-vinyl alcohol copolymer composition is taken out of the vessel through its lower part with the alcohol being let out through its upper part along with water vapor.
Also preferably, the amount of water vapor to be introduced into the vessel is between 0.3 and 30 times in terms of the ratio by weight to the amount of the ethylene-vinyl alcohol copolymer solution introduced thereinto. Also preferably, the temperature in the vessel in which the ethylene-vinyl alcohol copolymer solution is contacted with water vapor is between 100xc2x0 C. and 150xc2x0 C. and the pressure therein is between 1 and 6 kg/cm2.
Also preferably, the alcohol to be used in the production method is methanol. Also preferably, the ethylene content of the ethylene-vinyl alcohol copolymer is between 3 and 70 mol % and the degree of saponification thereof is at least 80 mol %.
Another aspect of the invention is ethylene-vinyl alcohol copolymer pellets which are produced by cutting the aqueous ethylene-vinyl alcohol copolymer composition obtained in the production method. For these, preferably, a melt of the aqueous ethylene-vinyl alcohol copolymer composition is extruded into strands in a coagulating bath and the thus-coagulated strands are cut. Also preferably, a melt of the aqueous ethylene-vinyl alcohol copolymer composition is directly cut.